In known turbochargers, the turbine stage comprises a turbine chamber within which a turbine wheel is mounted, an annular inlet passageway arranged around the turbine chamber, an inlet arranged around the inlet passageway, and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine chamber. A turbine wheel with radially extending blades is mounted in the turbine chamber and is rotated by the gas.
It is also well known to trim turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel.
Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passageway can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suit varying engine demands. In the most common type of variable geometry turbine each vane is pivotable about its own axis extending across the inlet passageway (typically aligned with a point approximately halfway along the length of the vane) and a vane actuating mechanism is provided which is linked to each of the vanes and is displaceable in a manner which causes each of the vanes to pivot in unison so that the trailing edge of each vane (i.e. that edge closest the turbine wheel) moves towards or away from an adjacent vane to vary the cross-sectional area available for the incoming gas as well as the angle of approach of the gas to the turbine wheel. Such arrangements are generally referred to as swing vane variable geometry turbines.
In another common type of variable geometry turbine, one wall of the inlet passageway is defined by a moveable wall member, generally referred to as a nozzle ring, the position of which relative to a facing wall of the inlet passageway is adjustable to control the width of the inlet passageway. For instance, as the volume of gas flowing through the turbine decreases the inlet passageway width may also be decreased to maintain gas velocity and optimise turbine output. In some cases the nozzle vanes are fixed in position but extend through slots in a moveable nozzle ring and in others the vanes extend from a moveable nozzle ring into slots provided on the facing wall of the inlet passageway.
In variable geometry turbines with a movable nozzle ring, it is known to provide for “over-opening” of the nozzle ring by withdrawing it beyond the nominal full width of the inlet passageway to retract the vanes at least partially from the inlet passageway and thereby increase the maximum inlet passageway flow area and gas flow rate. In a modification of this system, it is also known to provide a cut-out at the end of the nozzle vanes remote from the nozzle ring. This reduces the length of the trailing edge of the nozzle ring and the height of the nozzle vane over a portion of its width (the height of the vane being the distance it extends from the nozzle ring). There is thus a region at the end of each vane which has a reduced width and which is brought into the inlet passageway as the nozzle ring is over-opened to increase the area of the inlet passageway.
Whatever the form of the turbine, the nozzle vanes are stationary in the sense that they do not rotate with the turbine wheel. This leads to a well known problem caused by the interaction of the rotating wheel blades with a stationary pressure field resulting from the nozzle ring. That is, the periodic nature of this interaction can, at certain rotational speeds, correspond to the resonant frequency of the blades in one or more of their modes of vibration and set up oscillations in the blades.
It is an object of the present invention to obviate or mitigate the above problem.